The present invention relates to the exploration/production of a subterranean formation penetrated by a wellbore. More particularly, the present invention relates to techniques for communicating between equipment at the surface, and a downhole tool positioned in the wellbore.
The exploration and production of hydrocarbons involves placement of a downhole tool into the wellbore to perform various downhole operations. There are many types of downhole tools used in hydrocarbon reservoir exploration/production. Typically, a drilling tool is suspended from an oil rig and advanced into the earth to form the wellbore. The drilling tool may be a measurement-while-drilling (MWD) or a logging-while-drilling (LWD) tool adapted to perform downhole operations, such as taking measurements, during the drilling process. Such measurements are generally taken by instruments mounted within drill collars above the drill bit and may obtain information, such as the position of the drill bit, the nature of the drilling process, oil/gas composition/quality, pressure, temperature and other geophysical and geological conditions.
In some instances, it may be desirable to obtain additional data from the wellbore after drilling is complete. In such cases, the downhole drilling tool may be removed, and a separate downhole evaluation tool, such as a wireline, slickline, drill stem test or coiled tubing tool, may be lowered into the wellbore to perform additional testing, sampling and/or measuring. Downhole evaluation tools may be lowered into the wellbore via a conveyor, such as wired cable, drill pipe, slickline or coiled tubing. The evaluation tool may be withdrawn by its conveyor, or detached and left in the wellbore for later retrieval. Downhole drilling and/or measurement tools may be provided with communication systems adapted to send signals, such as commands, power and information, between a downhole unit housed in the downhole tool, and a surface unit. Communication systems in drilling tools may include, for example, mud pulse systems that manipulate the flow of drilling mud through a downhole drilling tool to create pressure pulses. One such mud pulse system is disclosed in U.S. Pat. No. 5,517,464 and assigned to the assignee of the present invention.
Communication systems in evaluation tools are typically incorporated into the conveyor itself, for example via wireline cable, wired drill pipe or wired coiled tubing. In such cases, wiring is usually provided in the conveyor to transmit signals between the surface and the downhole tool. The conveyor is often a wireline or armored set of insulated electrical cables which provides power to the tool, and controls the operation of the tool. The conveyor is often used to provide a wired communications link for the telemetry of signals between a surface control system (such as a computer) and the downhole tool. The tensile strength of the wireline cable may be a limit to the weight of the downhole tool and/or the physical and mechanical severity of the borehole conditions in which a downhole tool can be deployed.
In some instances, the communication system is unavailable, inactive or detached, such as during memory mode logging. In such situations, data is collected and stored in a memory unit within the downhole tool for later retrieval. By way of example, some wireline tools are deployed into the wellbore without the wireline connection between the surface system and the downhole tool. The use of a wireline can be too risky to use, or too costly to justify the expense. In such a case, the wireline cable is detached and the logging tool operates using self-contained power supplies (usually batteries) and data memory units (data memory and circuitry to bus the data from the sensors). Such a tool is placed in operation at the surface, then lowered into the wellbore by a conveyor, or dropped or pumped down the wellbore. The tool may be moved past multiple depth intervals, or it may be left at a single depth in the well. Regardless, the tool will record well data and store the data in memory for collection by the operator at some future time, such as when the tool is returned to the surface. During this type of “memory mode” logging, the operator typically has no communication with the tool to ensure that the tool is working properly throughout the operation, to turn the tool off and on, to change the type of data collected by the tool, or to change the frequency at which the data is collected.
The data collected during memory mode logging is typically retrieved by establishing (or re-establishing) a wired or mud pulse communication link between the downhole tool and the surface, or by retrieving the tool to the surface and downloading the information from the memory unit. While such techniques provide ways to capture downhole data stored in a downhole tool, there remains a need for techniques which facilitate the transmission of such data to the surface. It is desirable that such techniques utilize wireless technology to transfer the data. It is further desirable that a system be provided that is capable of providing a wireless telemetry link, while also providing many of the benefits of a wireline cable.
Wireless communication techniques, such electromagnetic (or emag) telemetry systems, have been employed in downhole drilling tools. Such systems include a downhole unit that creates an electromagnetic field capable of sending a signal to a remote surface unit. Examples of electromagnetic telemetry system are disclosed in U.S. Pat. Nos. 5,642,051 and 5,396,232, both of which are assigned to the assignee of the present invention. Current electromagnetic telemetry systems have been used in conventional MWD type drilling operations. It would be desirable to develop an electromagnetic telemetry system for evaluation tools.
Advancements, such as the use of repeaters and gaps, have been implemented in existing drilling tools to improve the operability of electromagnetic systems in drilling applications. By creating a gap, or non-conductive insert, between adjoining sections of drillpipe, the electromagnetic field is magnified and provides an improved signal. Examples of a gap used in an electromagnetic telemetry system are described in U.S. Pat. No. 5,396,232, assigned to the assignee of the present invention and U.S. Pat. No. 2,400,170 assigned to Silverman.
Electromagnetic telemetry systems incorporating gaps have been vulnerable to the leakage of drilling fluids into the circuitry mounted in drilling tools, resulting in failure of the electromagnetic telemetry system. Wellbore fluids tend to leak into the gap and between adjacent drill collars as well as into the electromagnetic circuitry housed in the drill collar. In an attempt to reduce such leakage, attempts have been made to install insulation into and between drill collars. Some examples of patents employing insulation in an electromagnetic system include U.S. Pat. No. 5,138,313 to Barrington, PCT Application No. 03/004826 to Frasier et al. and U.S. Pat. No. 4,348,672 to Givier. U.S. Pat. Nos. 6,098,727 and 6,493,324 to Ringgenberg et al. describe additional insulation technique involving coating threads of adjacent drill collars to create an insulating joint.
Existing insulation techniques have been vulnerable to damage in the harsh wellbore environments. Additionally, the insulation techniques have created problems in establishing proper mating between drill collars. Often, the insulation prevents the torquing necessary to create a sufficient connection between the drill pipes. To overcome such problems, jam nuts and epoxys have been used between adjacent drill pipes in an attempt to generate the required connection. However, there remains a need to provide improved insulation techniques to prevent failures in such gap systems. It would be desirable that such a system be capable of performing under harsh wellbore conditions, provide signals with reduced attenuation and/or provide stronger signals capable of operating in a variety of conditions.